1. Field of the Invention
The present invention relates to asynchronous transmission networks, notably networks of the so-called ATM (asynchronous transfer mode) type.
In these networks, digital information elements are transmitted discontinuously in the form of packets, called "information cells" in ATM terminology. This mode of transmission consists, instead of an octet-by-octet transmission, in constituting a packet of bits in the transmitter terminal equipment (the data source) as and when the digital data are created, and in waiting to constitute a complete information cell and in sending it to the network.
The transmitter equipment thus produces information cells at a variable rate, depending on whether or not it uses the channel (for example no cell is transmitted during the instants of silence) or at the variable rate at which the digital information elements are produced (for example in the typical case of the differential video signals, the data volume of which depends on the variably movable character of the image to be transmitted).
The information cell that will transit on the network comprises firstly a header that carries an address or "virtual channel identifier" and, secondly, a useful field that is a carrier of the information to be transmitted.
The information cells coming from several transmitters are mixed in a continuous flow, possibly with interposed empty cells that transit along high bit rate arteries.
The different arteries of the network connect a plurality of nodes, each corresponding to a switching device that provides for the demultiplexing and multiplexing of the flow of cells as a function of the address contained in the header and of the resources available at the instant considered.
The network is asynchronous in that each node of the network has a particular clock having its own frequency (and hence its drift) without any coordination for the mutual adjustment of the clocks of the different nodes.
2. Description of the Prior Art
To take account of the inevitable differences in frequencies, buffer memories provide for the adjustments between the input and output rates of each node. It is seen that, in principle, the network is always furnishes the rate to the terminals connected to it.
The particular feature of an ATM type network relates to the fact that the switch restricts itself to managing only the header of each information cell in determining the destination address from the header of the incoming cell and in computing, from this parameter, a new header that corresponds to the (virtual) channel that will have been assigned to the switched-over artery downline from this node.
In particular, no verification (such as an intrinsic error check, acknowledgment protocol etc.) is carried out on the data transmitted, the checks if any being transferred to the two ends of the line, i.e. to the two devices that exchange the information which, therefore, have to check the integrity and conformity of the information elements exchanged.
In general, a service corresponds to a transfer of information between two devices, each connected to a node providing access to the network, hence capable of having a difference in rate, the respective local clocks of each of these nodes being neither synchronous nor synchronized.
Services with a duration limited to a few minutes can generally make do with this absence of synchronization between nodes, given the precision and the stability of the clocks used, which is of the order of 10.sup.-8 at least.
However, in certain situations, the users may have to exchange synchronous information, either because they use synchronous terminals or because the asynchronous network is in fact only a sub-assembly of a more extensive network, including for example a synchronous switch to which the asynchronous network has to be connected.